This invention relates to dyes useful as fluorescent materials. More specifically, it relates to proton-transfer, low self-absorbing dyes particularly useful in scintillating detectors for high-energy radiation and particles. The dyes comprise novel chromophores exhibiting properties which enhance the performance of such scintillating detectors.
The detection of high-energy radiation can be accomplished through the use of compounds which scintillate (emit light) when a particle of radiation impinges on, or passes near, such compounds. Organic molecules capable of light emission based on fluorescence are called fluors, or chromophores. In the process of fluorescence, a chromophore is excited by absorbing an energy source, such as a photon, and then emits a photon of lower energy (longer wavelength) upon relaxation. Excitation of chromophores can also be produced by radiationless transfer of energy or by other high energy processes. Thus, the ability of chromophores to scintillate in this manner makes them a useful material for the detection or tracking of ionizing particles.
In current technological applications, chromophores are typically dispersed in a plastic medium, such as polystyrene. The term "scintillator" is applied to the polymer/chromophore ensemble.
A particularly useful chromophore is 3-hydroxyflavone (3HF). 3HF has the formula: ##STR3## 3HF is the chromophore of choice for many commercial scintillators because it emits a significantly longer wavelength than it absorbs. On excitation, a proton is transferred to the carbonyl group of the 3HF molecule in the following manner: ##STR4##
This property, known as proton-transfer fluorescence, produces a greater Stokes shift (the difference in absorbed and emitted wavelengths) for the 3HF molecule than occurs in other chromophore molecules. This enhanced Stokes shift is commercially significant in several respects. First, it bypasses radiation-induced color centers in the plastic medium which attenuate the light output of the scintillator and shorten its useful life. Second, 3HF's greater Stokes shift makes it a low self-absorbing fluor, since photons emitted at greatly reduced wavelengths are much less likely to be reabsorbed by another chromophore before exiting the scintillator.
While scintillators utilizing 3HF molecules dissolved in a plastic matrix are preferable to alternative scintillating materials for the reasons discussed above, they are not without shortcomings. For instance, there is a limit to the solubility of 3HF in plastics. Typically, scintillators using 3HF chromophores are limited to a chromophore concentration of about 1.2% (by weight at room temperature). This restricts the maximum level of brightness to which 3HF scintillators are capable. Further, over time, scintillators using chromophore molecules dispersed in a plastic medium are subject to chromophore migration and phase separation. This phenomenon adversely affects the quality of the scintillation produced by the material and reduces the useful life of the scintillator.
Scintillators comprising 3HF molecules are also limited as to other features important in detectors for high-energy radiation, such as their extinction coefficients, their quantum efficiency and their ability to red-shift emissions.